after you use it, how long you have to wait until de blueberries respawn again?
The bushes takes 1hr to respawn.
And where are the best places to farm it for the achievement "Bluebarian"?
I think that the best places are Isle of the kings and Greenshore, but if the Insectoid Invasion Mini World Change (Greenshore) isn't active, Greenshore is a GREAT place to complete this achievement.
Greenshore:
Carlin: Farm north west of Carlin
Isle of the kings: Behid the temple we use to go to the secret library
These are the 3 locations that I know has the most amout of blueberry bushes at Mainland.
Hope that this helps, have a great day! (>'.')>